I was wondering too, why the mods of civ4 haven't so much
new units....but i dont mean the vehicles of 1939 mods or maxriga
Because most people prefer to fight in the modern Era I find. Personally I like the early eras for warfare, its more..... desperate, hand to hand fighting has that true element of" if I don't put this pointy thing in him he will do it to me", while the modern era is like "pull this trigger and push this button, boom your dead", no fun

If your looking for new units, just wait until EBCIV is released, it will add probably around 500 units is its complete stage, not including the later expansions that will undoubtedly follow. And most important it will be based entirely in the classical era from 272 B.C.E to 14 AD.
Also keep in mind it takes a lot of effort to create new units from scratch, even more so when you are creating new animations (just look at Rabbit White's Centaur) The bones for people are more complex than for modern tanks, plane, and artillery too. Lastly, people with 3d modeling skills are at a premium here, sure there are some absolutely amazing unit creators here but they are at a premium compared with the demand of their time for many projects.